Move OMERO images out of the Git repository

- Remove the images/ folder under omero/
- Add a rsync-based `sync_images` script to download the images from necromancer

The new workflow for building the OMERO documentation should be as follows:
```
cd omero/
bash sync_images
make clean html latexpdf
```

`sync_images` is using `rsync` by default (with SSH authentication to necromancer). Alternately, `wget` can be used to download a zip of the images by passing the `-q` option

```
bash sync_images -q
```

To specify the documentation set, the `BRANCH` variable can be set, e.g. using final images set

```
cd omero/
BRANCH=dev_4_4 bash sync-images && make clean html latexpdf
```